1. Field of the Invention
The present invention relates to a method for fabricating a cathode in a cathode ray tube, and more particularly, to a method for fabricating a cathode in a cathode ray tube, in which a heat radiative material coating is applied to a metal wire, and the metal wire is inserted and passed through an opening of a cylindrical cathode sleeve and heated by providing a voltage thereto, to deposit the heat radiative material on an inside wall of the cathode sleeve, for improving electron emission characteristics of the cathode.
2. Background of the Related Art
Recently, cathode ray tubes, such as color picture tubes, have employed impregnated type cathodes for high current density which have been used in electron tubes, such as traveling wave tubes and klystrons, to cope with the trend of providing high definition pictures and large sized screens.
Referring to FIG. 1, the impregnated type cathode is provided with a cathode body 1 of porous refractory metal having an electron emission material, such as barium, calcium, aluminate, and etc., impregnated therein, a cylindrical cathode cup 2 having one side closed and the cathode body 1 inserted therein, a cylindrical cathode sleeve 3 having both ends opened and the cathode cup 2 inserted into an opening thereof starting from the closed side, and a heater 4 inserted in the cathode ray tube 3 for heating the cathode. The cathode sleeve 3 is supported. coaxially on a central portion of a cylindrical holder 5 having both ends opened spaced by three ribbons 6. Any of the cathode cup 2, the cathode sleeve 3, and the ribbons 6 are formed of a metal or alloy having at least one of tantalum Ta, niobium Nb, and molybdenum Mo as a major composition. And, there is a heat radiative coating 7 of tungsten W and alumina composition on an inside wall of the cathode sleeve 3 for making the cathode sleeve 3 to absorb a heat generated from the heater 4 efficiently. For example, the cathode sleeve 3 has a form of thin and long pipe with a 15xcx9c20 xcexcm thickness, 1.23 mm in diameter, and 4.3 mm in length. For example, as disclosed in Japanese Laid Open Patent H7-65714, the heat radiative coating 7 may be formed on the inside wall of the cathode sleeve 3 by disposing a 15 mm thick cathode sleeve vertically, inserting a fore end of a nozzle of an injector into the cathode sleeve a little from a top side of the cathode sleeve, injecting and removing a suspension of a mixture of required quantity of tungsten powder and alumina powder, thereby adsorbing the coating materials on the inside surface of the cathode sleeve. Then, the cathode sleeve the coating materials adsorbed thereto is heated at 1500xc2x0 C. for 10 min. under a vacuum, to form the heat radiative coating of approx. 5 xcexcm thickness. However, the Japanese Laid Open Patent H7-65714 has difficulty in that composition of the suspension is kept uniform owing to continuous vaporization of solvent from the suspension in the injector and a difference of densities of tungsten particles and alumina particles in a case a plurality of inside walls of the cathode sleeves are heat radiative coated. Consequently, it is liable that there can be deviations in a blackness and heat radiation of the coating on the inside wall of the cathode sleeve, and since the 15xcx9c20 xcexcm thick cathode sleeve is susceptible to embrittlement during the heat treatment at 1500xc2x0 C. for 10 min. in a vacuum ambient, the cathode sleeve may be broken in the next fabrication process of cathode assembly.
Moreover, though the purpose of the heat radiative coating is to make a heat generated at the heater to be absorbed by a circumference of the cathode sleeve, the heat radiative coating may fall off in the middle of operation or involved in a great heat loss with a consequential poor thermal efficiency if the heat radiative coating is thick. Therefore, the thickness of the heat radiative coating should be thinner for prevention of coating fall off and reducing a heat loss.
Accordingly, the present invention is directed to a method for fabricating a cathode in a cathode ray tube that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for fabricating a cathode in a cathode ray tube, which has a uniform heat radiation rate, can suppress embrittlement of the cathode sleeve, and permits to coat a plurality of inside walls of cathode sleeves on the same time.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the method for fabricating a cathode in a cathode ray tube, the cathode having a cathode sleeve with a heat radiative coating layer formed on an inside wall thereof for emission of thermal electrons, the heat radiative coating layer being formed by a method including the steps of (a) coating a material of the heat radiative coating layer on a surface of a metal wire provided separately, (b) inserting the metal wire having the heat radiative coating layer coated thereon, to pass through an opening the cylindrical cathode sleeve, and (c) heating the metal wire by providing a power thereto, to deposit the material of the heat radiative coating layer coated on the metal wire on the inside wall of the cathode sleeve.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.